chore(browsers): fix failing tests in Firefox

Closes #3386

modules/angular2/src/dom/browser_adapter.dart

@@ -188,6 +188,9 @@ class BrowserDomAdapter extends GenericBrowserDomAdapter {
   void preventDefault(Event evt) {
     evt.preventDefault();
   }
+  bool isPrevented(Event evt) {
+    return evt.defaultPrevented;
+  }
   String getInnerHTML(Element el) => el.innerHtml;
   String getOuterHTML(Element el) => el.outerHtml;
   void setInnerHTML(Element el, String value) {

modules/angular2/src/dom/browser_adapter.ts

@@ -104,6 +104,9 @@ export class BrowserDomAdapter extends GenericBrowserDomAdapter {
     evt.preventDefault();
     evt.returnValue = false;
   }
+  isPrevented(evt: Event): boolean {
+    return evt.defaultPrevented || isPresent(evt.returnValue) && !evt.returnValue;
+  }
   getInnerHTML(el): string { return el.innerHTML; }
   getOuterHTML(el): string { return el.outerHTML; }
   nodeName(node: Node): string { return node.nodeName; }

modules/angular2/src/dom/dom_adapter.ts

@@ -43,6 +43,7 @@ export class DomAdapter {
   createMouseEvent(eventType): any { throw _abstract(); }
   createEvent(eventType: string): any { throw _abstract(); }
   preventDefault(evt) { throw _abstract(); }
+  isPrevented(evt): boolean { throw _abstract(); }
   getInnerHTML(el): string { throw _abstract(); }
   getOuterHTML(el): string { throw _abstract(); }
   nodeName(node): string { throw _abstract(); }

modules/angular2/src/dom/html_adapter.dart

@@ -102,6 +102,9 @@ class Html5LibDomAdapter implements DomAdapter {
   preventDefault(evt) {
     throw 'not implemented';
   }
+  isPrevented(evt) {
+    throw 'not implemented';
+  }
   getInnerHTML(el) {
     return el.innerHtml;
   }

modules/angular2/src/dom/parse5_adapter.ts

@@ -154,6 +154,7 @@ export class Parse5DomAdapter extends DomAdapter {
     return evt;
   }
   preventDefault(evt) { evt.returnValue = false; }
+  isPrevented(evt): boolean { return isPresent(evt.returnValue) && !evt.returnValue; }
   getInnerHTML(el): string { return serializer.serialize(this.templateAwareRoot(el)); }
   getOuterHTML(el): string {
     serializer.html = '';